Stone County Sheriff Doug Rader said that the boat sank to the bottom of the lake and that seven passengers were taken to a hospital. Divers were searching for missing passengers, and the sheriff said that would continue overnight.

A sheriff’s deputy was among those on the boat when it capsized and was assisting in the rescue, he said.

The boat capsized because of severe weather, Rader said, adding that he believed that all the fatalities were caused by drowning.

The Southern Stone County Fire Protection District said on Facebook that 20 or more people were on the boat.

Ripley Entertainment owns the boat, having acquired the Ride the Ducks attraction in Branson last year. Duck boats can float on the water and drive on land.

Suzanne Smagala-Potts, a spokeswoman for Ripley Entertainment, said this was the first time an accident had happened at this location. “Our thoughts are first and foremost with the families,” she said.

Family members of missing people believed to have been on the boat were directed to go to Branson City Hall.

Steve Lindenberg, a meteorologist at the National Weather Service’s office in Springfield, Missouri, about 45 miles north of Branson, said a line of thunderstorms rattled the area Thursday night and produced winds of up to 74 mph. The winds downed trees and power lines, he said.

Lindenberg said a 63-mph gust was recorded at Branson’s airport around 6:55 p.m. local time, although he did not know whether the winds had caused the boat to capsize.

He said the thunderstorms had since left Missouri and moved into Arkansas.

Rod Donavon, a National Weather Service meteorologist in Des Moines, Iowa, said several tornadoes swept through the central part of that state Thursday, apparently damaging a warehouse, homes and other structures.

Two, in the cities of Pella and Marshalltown, struck within about 30 minutes of each other and were particularly destructive, Donavon said. He described them as “strong” but said the exact strength of the winds were not yet clear.